Problem: If $a + b = 3$ and $x + y + z = -7$, what is $-6z - 7b - 6y - 6x - 7a$ ?
Explanation: $= -7a - 7b - 6x - 6y - 6z$ $= (-7) \cdot (a + b) + (-6) \cdot (x + y + z)$ $= (-7) \cdot (3) + (-6) \cdot (-7)$ $= -21 + 42$ $= 21$